What Is A Mineral?: Mineralogy

What is a Mineral? Mineralogy How Minerals are formed? JN Malik

Major Locations where Magma come out from Volcanoes

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Following definitions of mineral from different sources:

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"A mineral is a naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement" (Mason et al., 1968).

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"A mineral is a body produced by the processes of inorganic nature, having usually a definite chemical composition and, if formed under favorable conditions, a certain characteristic atomic structure is expressed in its crystalline form and other physical properties" (Dana & Ford, 1932).

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"Minerals are naturally-occurring inorganic substances with a definite and predictable chemical composition and physical properties" (Donoghue, 1990).

 A mineral is a natural, inorganic, homogeneous solid substance having a definite chemical composition and regular atomic structure

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 This suggests that the mineral must have formed naturally under natural condition and not artificially or synthetically (made by man in lab) e.g., Artificial diamonds (American diamond); rubies, sapphires and emeralds etc., these cannot be called minerals.

Diamond is the hardest natural mineral known to mankind

Ruby is the red (Latin "Rubrum," meaning red) variety of corundum, the second hardest natural mineral known to mankind. In Sanskrit – “Ratanraj”

Sapphire is the non-red variety of corundum

v Coal, petroleum, natural gases etc., are organic substances which to strict sense are not minerals as per the definition; however, they are considered as mineral and called FUEL MINERALS

CRYSTAL: Greeks called krystallos; the Romans latinized as crystallum.  The word crystal can be applied to any solid substance that grows with planar

surface or a naturally occurring solid body bounded by smooth and plain surface arranged in an geometrical pattern

 This is an outer expression of internal atomic structure.

Si

O JN Malik

A plain surface that bounds a crystal is called CRYSTAL FACE, and the geometric arrangement of crystal faces is called CRYSTAL FORM. Crystal face

Fluorite Most common habit is hexagonal prisms terminated with a six sided pyramid

Crystal habit is Cubic or Isometric

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Mineraloid • Some naturally occurring solid compounds do not meet the definition of a mineral because they lack: • a definite composition, or • a characteristic crystal structure, or • both.

• e.g., Opal: is amorphous in nature JN Malik

Polymorphs: – Minerals having same chemical composition but different crystal structure: – Calcite (CaCO3; Hexagonal) – Aragonite (CaCO3; Orthorhombic)

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Conditions for formation of Crystals: • Slow cooling of magma • Free surroundings that facilitate the crystal growth in different directions

• No interference of the adjacent or nearby growing minerals during solidification.

Mode of formation of minerals There are three mode of mineral formation (a) by cooling of Magma (b) by secondary processes

(c) under metamorphism

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Mode of formation of minerals a) Most of the minerals are formed during different stages of cooling of magma. e.g., 90% of rock-forming minerals like Feldspar, Quartz, Amphiboles, Micas and Olivine. Also some precious minerals Gemstones like- Garnet, Topaz, Beryl, Tourmaline etc. are formed during cooling of magma (Magmatic Crystallization) b) Secondary processes like weathering, precipitation and deposition leads to formation of minerals like Calcite, Dolomite, Bauxite; Salt, Coal, Petroleum, Chlorite, Clay minerals - Kaolinite (china clay), Semectite, Montmorillonite, Agate, Opal etc. c)

By metamorphism the minerals are formed under the influence of high temperature and pressure with and without involvement of chemically active solution, e.g., Andalusite, Sillimanite, Kyanite, Staurolite, Garnet, Graphite etc.

Major Locations where Magma come out from Volcanoes

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Lava flow on the surface and slowly cools down giving rise to different mineral of different composition

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IMPROTANT ROCK FORMING MINERALS 

More than 70% of earth crust is made up of element like Oxygen and Silicon.  In order of abundance, Silicates - are the most abundant rock forming minerals, followed by Oxides, Carbonates, Phosphates, Sulphates etc

• SILICATES MINERALS: Silicate minerals are the minerals, which comprise Silicate tetrahedronsilicate anion (SiO4)-4 e.g., Feldspars; Pyroxenes; Amphiboles; Olivine; Garnet; Quartz; Clays; Mica

• NON-SILICATE MINERALS: O2=Oxides: Hematite (Fe2O3); Magnetite (Fe3O4) S-2=Sulfides: Galena (Pbs); Chalcopyrite (CuFeS2) (CO3)-2=Carbonates: Calcite (CaCO3) Sulfates= (SO4)-2 :Gypsum (CaSo4.2H2O)

Phosphates= (PO4)-3 : Apatite JN Malik

FELDSPARS

Orthoclase (Monoclinic) and Microcline (Monoclinic) KAlSi3O8

Albite (Triclinic) Na AlSi3O8

Plagioclase

Labradorite (Triclinic) (NaCa)AlSi3O8

Anorthite (Triclinic) CaAl2Si2O8

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Orthoclase

Microcline

Albite

Labradorite JN Malik

Anorthite JN Malik

PYROXENES:

Augite

Ca(Mg,Fe,Al) (Si Al)2 O6

Jadite/Jade

Na Al Si2 O6

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AMPHIBOLE : Mg7Si8O22 (OH)2

Hornblende

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OLIVINE (Mg, Fe)2 SiO4

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MICA Muscovite

Biotite KAl2(Si3 Al)O10 (OH)2

K(MgFe)3 AlSi3 Al O10 OH)2

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QUARTZ : SiO2

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Non-Silicate Minerals

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Bowen in (1922) identified two series of mineral formation/crystallization. He suggested that Minerals crystallize in a particular manner with varying temperature ranging from 1400C to 800C

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Discontinuous Series –

–

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Mineral crystallize in sequence with decreasing temperature, mainly Ferro-Magnesium minerals e.g., Olivine, Pyroxene, Amphibole, Biotite). If one mineral becomes chemically unstable, another mineral begins to form

Continuous branch –

Plagioclase feldspar forms with a chemical composition that evolves from Ca-rich to Na-rich with decreasing temperature

BOWEN REACTION SERIES Discontinuous Series

14000C

Continuous Series Non Ferro-Magnesium

Ferro-Magnesium

(Feldspar) Minerals

(Mafic) minerals Olivine (isolate SiO4) (Mg, Fe)2 SiO4

Ca-Plagioclase (Anorthite)

CaAl2Si2O8 Pyroxene (Single chain) (eg. Augite, Hypersthene) Ca(Mg, Fe, Al) (Si Al)2 O6

Na-Ca Plagioclase (eg. Labradorite)

NaCaAlSi3O8

Amphibole (double chain) (Mg7Si8O22 (OH)2 eg. Hornblende)

Na- Plagioclase (eg. Albite)

Na AlSi3O8

Bitotie K(MgFe)3 AlSi3 Al O10 OH)2

Potash Feldspar (eg. Orthoclase) KAlSi3O8 Muscovite

8000C

Quartz

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CRITERIA FOR IDENTIFICATION OF MINERALS 1) CRYSTAL SYMMETRY: Most of the minerals occur in nature are in crystalline form, which is a reflection of their respective Internal Atomic Arrangement. 2) This geometrical arrangement of atoms result into different shapes of crystal e.g. Cubic; Prism; Hexagonal etc. 3) On the basis of symmetry and crystal form, these forms are subdivided into six systems:

The Six Crystal Systems •Cubic: a = b = c; alpha = beta = gamma = 900. •Tetragonal: a = b  c; alpha = beta = gamma = 900. •Hexagonal: a = b  c; alpha = beta = 1200; gamma = 900. •Orthorhombic: a b  c; alpha = beta = gamma = 900. •Monoclinic: a  b c; alpha = gamma = 900; beta > 900. • Triclinic: a  b  c; alpha and beta > 900; gamma . JN Malik

eg. Halite; Garnet; Pyrite, Fluorite

eg. Olivine; Staurolite; Topaz

eg. Beryl; Quartz; Calcite; Hematite

eg. Augite; Orthoclase; Gypsum; Mica; Clay min.; Hornblende

eg. Zricon

eg. Plagioclase Feldspar; Kyanite;

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Isometric (crystal system) - Three axes, all of them are equal in length and lie at 90° from the other.

Hexagonal (crystal system) - Four axes, three are equal in length and lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners.

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Tetragonal (crystal system) Three axes, two are equal in length, one is unequal. All three axes are at 90° to each other.

Orthorhombic (crystal system) - Three axes, all are unequal in length. All three axes are at 90° to each other.

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Monoclinic (crystal system) Three axes, all of them are unequal in length. Two of them are at right angles to each other, while the third is lies at an angle other than 90°.

Triclinic (crystal system) - Three axes, all of them are unequal in length, none of them are right angles to each other.

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• The simpler terms used to describe cleavage are: perfect, imperfect, good, distinct, indistinct, and poor. • Cleavage is said to be BASAL when it occurs perpendicular to the major axis of the mineral and •PRISMATIC when it occurs parallel to the major axis. •Multiple cleavages that produce geometric polygons are referred as OCTAHEDRAL cleavage in the mineral fluorite, •CUBIC cleavage in the mineral halite or

•RHOMBOHEDRAL cleavage in calcite • One Set of cleavage- Mica, Chlorite, Talc • Two sets- Feldspars, Pyroxenes, Amphiboles • Three sets- Calcite, Dolomite, Galena or • Four sets- Fluorite JN Malik

Cube comprise 6 Square faces

Composed of 8 equilateral triangular faces

12 rhomb-shaped faces

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What all information we need for mineral identification

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4) COLOUR: It is due to the individual mineral. Eg. ColourlessWhiteRedBlueGreenBlackYellow-

chemical composition of Quartz, Calcite, Gypsum Orthoclase, Muscovite Ruby, Garnet, Japer Kyanite Olivine, Emerald Hornblende, Tourmaline Topaz, Sulphur, Pyrite Augite

Ca(Mg,Fe,Al) (Si Al)2 O6

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•7) STREAK: is the colour of the powder of a mineral. When a mineral is rubbed on unglazed porcelain plate it give a line of colour.

• Garnet is red colour • Epidote is Green colour • Augite is Dark Green to Black • Hematite is Black to Reddish Brown• Olivine is Green -

Streak is colourless Colourless Greenish Gray Red White to Colourless

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5) LUSTER: Luster is the natural shining on the surface of the mineral. The appearance of mineral surface under reflected light is referred as its luster. VitreousLuster of a broken glass SilkyLuster of silk Pearlyshine like pearl Adamantine- shine of diamond Metallicshine of Iron/metal

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3) HARDNESS: Hardness of the mineral can be known by rubbing the mineral or the material listed below How to know the hardness?

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• 6) SPECIFIC GRAVITY: It is the ratio of the weight of a substance to the weight of an equal volume of pure water. Specific gravity (density) of a mineral depends upon their chemical composition and atomic structure. • Minerals with Sp. Gravity < 2 feel light 2-4 feel normal >4 heavy

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2) CLEAVAGE: WHAT IS CRYSTAL CLEAVAGE? • A definite plane along which a mineral tends to break easily is called cleavage. • When a mineral breaks, it breaks either by fracturing or by

cleaving. Crystal cleavage is a smooth break producing a flat crystal face. • It is mainly related to nature of crystallinity, only the crystalline minerals can have cleavage. • Amorphous minerals like Bauxite does not have cleavage. • Depending upon their atomic structure the minerals will have different set of cleavage.

Halite (NaCl) JN Malik

Crystals of clay mineral (Kaolinite) under SEM

• Sheet Cleavage: Clay also has sheet cleavage, which enhance it capability of absorb water between the sheets making the wet clay weaker, slippery and easy to mold

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